1. Field of the Invention
The present invention relates to a beam scanning apparatus, and more particularly, to a beam scanning apparatus comprising a collimating lens consisting of one sheet of a spherical surface.
2. Description of the Related Art
Generally, a laser printer prints an image on printing paper using a laser beam scanning apparatus. The beam scanning apparatus focuses light beams emitted from laser diodes onto a photosensitive drum according to an image signal, thereby forming a latent image on the photosensitive drum. The latent image is printed on the printing paper in a visible image.
The beam scanning apparatus comprises a collimating lens for transforming the light beam emitted from the laser diode into a parallel beam or a convergent beam. In order to improve utilization of the light beam emitted from the laser diode, the collimating lens is designed to have a large numerical aperture (NA) which varies in value depending on a main scanning direction and a sub-scanning direction.
Conventional collimating lenses use an aspherical surface lens made of a plastic so as to achieve a compact-sized and cost-saving beam scanning apparatus. However, conventional collimating lenses vary in refractivity with temperature changes in the beam scanning apparatus, causing the image spot to change.
Also, conventional collimating lenses adjust a wavefront aberration through the combination of one sheet of a spherical surface lens and one sheet of an aspherical surface lens or through one sheet of aspherical surface glass lens. However, while such collimating lenses are capable of adjusting the wavefront aberration, they are accompanied with increased fabrication cost.